1. Field of the Invention
The present invention relates to a method of and apparatus for reading image information by the use of a stimulable phosphor sheet, and a solid-state image detector used therefor.
2. Description of the Related Art
It has been well known in the art to read out image information using a stimulable phosphor sheet. The stimulable phosphor stores part of radiation energy when exposed to radiation, and exhibits photostimulated luminescence (PSL) according to the stored energy when exposed to stimulating light, such as visible light, etc. The radiation image information of a subject, such as a human body, etc., is temporarily recorded on a stimulable phosphor sheet. The stimulable phosphor sheet is scanned with a stimulating light beam such as a laser beam, and is caused to emit photostimulated luminescence light. The photostimulated luminescence light is photoelectrically detected and converted to an image signal carrying the radiation image information. As the image information reading apparatus, a wide variety of apparatus varying in manner of scanning and in the form of the photoelectric conversion means have been proposed.
For example, Japanese Unexamined Patent Publication Nos. 55(1980)-12492, 56(1981)-11395, etc. (hereinafter referred to as Reference 1), disclose an image reading method and apparatus, which is equipped with a stimulating light source for emitting a light spot, such as a laser beam, etc., as stimulating light; a photomultiplier (zero-dimensional photoelectric converter) with an electron doubling function of converting the photostimulated luminescence light, emitted from a sheet by irradiation of the light spot, to an electrical signal; and a stimulating-light scanning optical system for irradiating the light spot onto the sheet in a horizontal-scanning direction and, at the same time, moving the stimulating light and the photomultiplier (including an optical guide) relative to the sheet on the sheet surface in the vertical-scanning direction approximately perpendicular to the horizontal-scanning direction. In the image reading apparatus, the photostimulated luminescence light, emitted from the sheet when scanned with the light spot, is sequencially read by the photomultiplier.
The photomultiplier as a photoelectric converter has a high sensitivity to the wavelength of photostimulated luminescence light ranging from about 300 to 500 nm (blue light band) and a low sensitivity to the wavelength of stimulating light ranging from about 600 to 700 nm (red light band). The photomultiplier amplifies a micro signal resulting from feeble photostimulated luminescence light, by the external photoelectric effect so that it is not influenced by electrical noise.
The photomultiplier may have a circular or polygonal photomultiplier and may be used along with a focusing guide in the form of a dustpan, or may be employed as a long photomultiplier which has a photomultiplier having approximately the same length as the sheet width (width in the horizontal-scanning direction). In either case the photomultiplier is used as a zero-dimensional detector.
The apparatus employing the photomultiplier, however, has the following problems:
1) The photomultiplier has low shock resistance, because it is constructed of a hollow glass tube.
2) The photomultiplier is fairly difficult to be made in a thin form, as it uses a complex multistage dynode to double photons. The cost of manufacturing a long photomultiplier which has a width of 17 inches would be unduly high.
3) The quantum efficiency of the photocathode utilizing external photoelectric effect is low. The quantum efficiency with respect to photostimulated luminescence light of wavelengths 300 to 500 nm (blue light band) is normally as low as about 10 to 20%, whereas the quantum efficiency with respect to photostimulated luminescence light of wavelengths 600 to 700 nm (red light band) is relatively great and normally about 0.1 to 2%. For this reason, a special stimulating light cut filter becomes necessary to obtain a satisfactory signal-to-noise ratio (S/N) and results in an increase in the manufacturing cost.
4) Since the photomultiplier uses a complex multistage dynode, it is difficult for the photomultiplier to constitute a long one-dimensional detector (line sensor) of 17 inches in width which has a small pixel size of about 100 xcexcm.
Japanese Unexamined Patent Publication No. 60(1985)-111568 (hereinafter referred to as Reference 2) has proposed an image reading method and apparatus to reduce the time for reading photostimulated luminescence light, make the apparatus compact in size, and reduce the cost. This apparatus is equipped with a linear light source (stimulating light source such as a fluorescent lamp, a cold cathode type fluorescent lamp, a light-emitting diode array, etc.) for irradiating stimulating light in a form of a line onto an image recording sheet having a stimulable phosphor layer; a line sensor (photoelectric converter) having a large number of solid photoelectric conversion elements arrayed along a line on the image recording sheet irradiated with linear light (stimulating light) by the linear light source, and scanning means for moving the linear light source and the line sensor relative to the sheet on the sheet surface in the vertical-scanning direction approximately perpendicular to the length-wise direction of the irradiated line. The photostimulated luminescence light, emitted from the sheet when scanned with the stimulating light, is sequencially read by the line sensor.
The Reference 2 discloses photoconductors as solid photoelectric conversion elements which constitute a line sensor used herein. Solid photoelectric conversion elements with bandgap E greater than the photon energy hc/xcex of stimulating light wavelength xcex (E greater than hc/xcex), and solid photoelectric conversion elements with bandgap E smaller than hc/xcex (E less than hc/xcex), are both used. As examples of substance having E greater than hc/xcex, there are ZnS, ZnSe, CdS, TiO2, ZnO, etc. As examples of substances having E less than hc/xcex, there are xcex1-SiH, CdS(Cu), ZnS(Al), CdSe, PbO, etc. Note that the Greek letter xe2x80x9cxcex1xe2x80x9d in xe2x80x9cxcex1-SiHxe2x80x9d means xe2x80x9camorphous.xe2x80x9d Reference 2 also proposes use of a line sensor composed of silicon photodiodes.
The apparatus that uses a line sensor employing a plurlity of substances mentioned above, however, has problems as follows. That is, as photostimulated luminescence light is feeble, a photoconductor to be used is required to have extremely high dark resistance. However, the disclosed photoconductors are all low in dark resistance. Since reading is performed under a relatively large electric field, the dark current increases and it is fairly difficult to obtain a satisfactory S/N ratio. Particularly, when the bandgap E is small (E less than hc/xcex), the dark current resulting from thermal excitation is large and therefore it is extremely difficult to obtain a satisfactory S/N ratio.
In addition, in order to facilitate manufacture, a substance, which can be manufactured at a low substrate temperature and become, even in a large area, uniform in characteristic, is preferred. However, the aforementioned substance must be manufactured at a high substrate temperature of 100 xc2x0 C. or greater. Because each substance is composed basically of two kinds of elements, a film, which stabilizes the composition and is uniform, even in a large area, in characteristic, is made and therefore results in an increase the in the manufacturing cost.
Japanese Unexamined Patent Publication No. 60(1985)-236354 (hereinafter referred to as Reference 3) proposes an image reading method and apparatus, which is equipped with a stimulating light source for emitting a light spot, such as a laser beam, as stimulating light; and a stimulating-light scanning optical system for irradiating a sheet with the light spot in a horizontal-scanning direction and, at the same time, moving the stimulating light and a line sensor relatively to the sheet on the surface of the sheet in the vertical-scanning direction perpendicular to the horizontal-scanning direction. The photostimulated luminescence light, emitted from the sheet when scanned with the light spot, is sequencially read by the line sensor. The solid photoelectric conversion elements that constitute the line sensor used herein, however, are the same as those disclosed in Reference 2 and therefore have the same problems as mentioned above.
xe2x80x9cRadiographic Process Utilizing a Photoconductive Solid-State Imager (772/Research disclosure, October, 1992/34264)xe2x80x9d (hereinafter referred to as Reference 4) discloses a system, which is provided with a radiation-image converting panel (which is one form of the photoelectric conversion means) as a zero-dimensional photoelectric converter. The radiation-image converting panel is constructed of an image recording sheet having a stimulable phosphor layer which emits photostimulated luminescence light by an amount corresponding to energy stored when irradiated with stimulating light, and a photoconductive layer (interposed between two electrode layers) having a sensitivity to the photostimulated luminescence light. The image recorded on the radiation-image converting panel is read by scanning the panel two-dimensionally with light spot. It is disclosed that for the photoconductive layer constituting the panel, one having a high sensitivity to a photostimulated luminescence light wavelength of 500 nm and a low sensitivity to a stimulating light wavelength of 633 nm is satisfactory and amorphous selenium (xcex1-Se) is preferred.
Amorphous selenium (xcex1-Se) is highly sensitive to a wavelength of 500 nm or less (e.g., a blue light band of about 300 to 500 nm), is high in quantum efficiency relative to the photostimulated luminescence light near wavelength 400 nm, compared with a photomultiplier as a zero-dimensional photoelectric converter, and results in an efficient combination with the stimulable phosphor layer which is suitable for reading the photostimulated luminescence light emitted from the stimulable phosphor layer, compared with the photoconductive layers disclosed in the aforementioned References 1 to 3. In addition, xcex1-Se is nearly insensitive to a wavelength of 600 nm or greater (e.g., a red light band of about 600 to 800 nm), is greater in ratio of a sensitivity to photostimulated luminescence light to a sensitivity to stimulating light, and is basically capable of detecting the photostimulated luminescence light emitted from the surface of the stimulable phosphor layer, without using a stimulating-light cut filter. Furthermore, xcex1-Se is suitable for solidification (e.g., it is high in shock resistance) and can be thinned and increased in area, because a low-temperature deposition process can be performed on xcex1-Se.
If the converting panel has approximately the same area as the sheet, however, the area of the photoconductive layer will become larger and therefore the manufacturing cost will be increased. Since the area of the photoconductive layer becomes larger, the generation of excessive dark current cannot be avoided, and since capacitance (output capacitance of the detector) also becomes greater, only an image with a poor S/N ratio can be obtained.
Japanese Patent Publication No. 7(1995)-76800 (hereinafter referred to as Reference 5), with the aforementioned Reference 4, discloses that the photostimulated luminescence light emitted from the stimulable phosphor layer is detected with the photoconductive layer of approximately the same area as an image detecting sheet. It also discloses that as an example of the photoconductive layer, one having a high sensitivity to photostimulated luminescence light wavelengths of 300 to 500 nm and a low sensitivity to stimulating light wavelengths of 600 to 800 nm is satisfactory, and particularly, a selenide is preferred. Furthermore, it discloses that the influence of dark current is reduced by dividing each of the electrodes provided on opposite sides of the photoconductive layer interposed therebetween, and detecting each current independently.
However, even if each electrode is divided, the area of the photoconductive layer will remain large and approximately the same area as the sheet, and the problem of increasing the manufacturing cost remains unsolved. In addition, since the total area of the electrodes remains large, the generation of excessive dark current cannot be avoided yet, and since capacitance is also great, the S/N ratio is not so improved.
Japanese Unexamined Patent Publication No. 58(1983)-121874 (hereinafter referred to as Reference 6), with the aforementioned References 4 and 5, discloses that the photostimulated luminescence light emitted from the stimulable phosphor layer is detected with the photoconductive layer of approximately the same area as an image detecting sheet and that a selenide is employed as the photoconductive layer. It also discloses that the influence of dark current is reduced by dividing each of the electrodes provided on opposite sides of the photoconductive layer interposed therebetween, and detecting each current independently. Furthermore, it discloses that in the case where the capacitance of the photoconductive layer is great and therefore additional noise will develop, each electrode is divided, for example, into parallel bands.
However, similarly to Reference 5, even if each electrode is divided, the area of the photoconductive layer will remain approximately the same area as the sheet and large, and the cost will be increased. In addition, because the total area of the electrodes remains large, the generation of excessive dark current cannot be avoided, and because capacitance is also great, the problem of a poor S/N ratio remains unsolved.
The present invention has been made in view of the aforementioned circumstances.
Accordingly, it is a first major object of the present invention to provide an image information reading method and an image information reading apparatus that are capable of obtaining an image with an improved S/N ratio, using a solid-state image detector.
A second major object of the invention is to provide, as photoelectric conversion means for detecting feeble photostimulated luminescence light emitted from an image recording sheet having a stimulable phosphor layer, a solid-state image detector which has great dark resistance and small output capacitance and is also high quantum efficiency relative to photostimulated luminescence light and a higher ratio of a sensitivity to blue light to a sensitivity to red light than a photomultiplier and capable of obtaining a satisfactory S/N ratio.
A third major object of the invention is to provide a solid-state image detector which is robust in shock resistance and is easy to thin and manufacture and low in cost.
To achieve the objects mentioned above, there is provided a method of reading image information, comprising the steps of:
using an image recording sheet which has a stimulable phosphor layer that emits photostimulated luminescence light of a quantity corresponding to energy stored when irradiated with stimulating light, and a solid-state image detector which has a photoconductive layer that exhibits conductibility when irradiated with the photostimulated luminescence light;
scanning the image recording sheet carrying the image information recorded thereon with the stimulating light;
guiding photostimulated luminescence light obtained by the scanning so that the photostimulated luminescence light is incident on the photoconductive layer;
detecting electric charge generated in the photoconductive layer by the incidence of the photostimulated luminescence light, under an electric field applied across the photoconductive layer; and
obtaining an image signal which carries the image information by detecting the electric charge;
wherein the stimulable phosphor layer of the image recording sheet is stimulated with the stimulating light having a wavelength of 600 nm or greater (preferably, a red light band of 600 to 800 nm) and emits the photostimulated luminescence light having a wavelength of 500 nm or less (preferably, a blue light band of 300 to 500 nm);
the photoconductive layer of the solid-state image detector contains amorphous selenium as its main component and also has a smaller area than that of the image recording sheet; and
the scanning is performed by moving the solid-state image detector relative to the image recording sheet on the surface thereof.
The expression xe2x80x9chas a smaller area than that of the image recording sheetxe2x80x9d is intended to mean that the length of each side of the solid-state image detector is shorter than or equal to that of each side of the image recording sheet in the directions corresponding to the aforementioned scanning and that at least one side is shorter than a side of the sheet. In the case of a long detector, for instance, the side in the longitudinal direction (horizontal-scanning direction) may be approximately the same as that of the sheet (which is preferable). However, the width is made narrower than the sheet width. In the case of a zero-dimensional detector approximately in the form of a square, each side is made smaller than the sheet.
It is preferable that in the image information reading method of the present invention, the length of at least one side of the solid-state image detector be one fifth or less of the length of one side of the image recording sheet. That is, it will be sufficient if the solid-state image detector has a slightly narrower width than the sheet length and enough size (lowest width) to detect photostimulated luminescence light.
It is also preferable that the thickness of the photoconductive layer of the solid-state image detector be 0.1 xcexcm or greater in order to sufficiently absorb photostimulated luminescence light and make the signal level greater. A thicker photoconductive layer is preferred in order to render distributed capacitance smaller so that fixed noise is suppressed. However, if the film thickness is too large, the power-supply voltage for applying an electric field must be increased. Therefore, in consideration of power-supply voltage and in order to make fixed noise smaller, it is desirable to use a solid-state image detector in which the thickness of a photoconductive layer thereof is between 0.1 and 100 xcexcm.
In a preferred form of the image information reading method of the present invention, the solid-state image detector comprises two solid-state image detectors disposed on opposite sides of the image recording sheet so that each detector can detect the photostimulated fluorescent light emitted from each surface of the image recording sheet.
In another preferred form of the image information reading method of the present invention, the solid-state image detector extends lengthwise in a horizontal-scanning direction scanned with the stimulating light, and comprises a plurality of solid-state image detectors disposed in parallel with the horizontal-scanning direction and along a vertical-scanning direction so that each detector can detect the photostimulated luminescence light emitted from the image recording sheet.
In the image information reading method of the present invention, it is desirable to dispose an optical filter between the photoconductive layer and the image recording sheet. For instance, the optical filter may be a stimulating light cut filter for cutting stimulating light and transmitting photostimulated luminescence light.
It is also desirable to apply an electric field under which avalanche amplification is obtained within the photoconductive layer. To effectively obtain avalanche amplification, it is desirable to employ a photoconductive layer having a thickness of 1 xcexcm or greater, preferably 10 xcexcm or greater. On the other hand, it is necessary to apply a high electric field across the photoconductive layer in order to obtain avalanche amplification, and if the film thickness is too large, power-supply voltage for applying an electric field must be increased. Therefore, in consideration of power-supply voltage and in order to effectively obtain avalanche amplification, it is desirable to use a solid-state image detector in which the thickness of a photoconductive layer thereof is within the range of 10 to 50 xcexcm.
Note that if a photoconductive layer containing amorphous selenium (xcex1-Se) as its main component is used under an electric field in which avalanche amplification is obtained, it will be sensitive to fluctuations in the electric-field distribution (e.g., due to fluctuations in power-supply voltage) and therefore the image signal will fluctuate. For this reason, it is preferable to suppress the fluctuations in the image signal induced by the fluctuations in the electric-field distribution. The method of suppression may be, for example, a method of suppressing power-supply voltage fluctuations to the utmost for stabilization of voltage, or a method in which, in addition to voltage stabilization, power-supply voltage fluctuation data on fluctuations in output data with respect to power-supply voltage fluctuations is acquired and stored, power-supply voltage fluctuations during image reading are also monitored, and an image signal is corrected, e.g., by software, according to the power-supply voltage fluctuations during image reading.
To achieve the aforementioned objects of the present invention, there is also provided an apparatus for reading image information, comprising:
a light source for emitting stimulating light;
stimulating-light scanning means for scanning an image recording sheet with the stimulating light, the recording image sheet having a stimulable phosphor layer which emits photostimulated luminescence light of a quantity corresponding to stored energy when irradiated with the stimulating light;
a solid-state image detector having a photoconductive layer which exhibits conductibility when irradiated with the photostimulated luminescence light;
voltage application means for applying voltage across the photoconductive layer so that an electric field is generated; and
image-signal acquisition means for obtaining an image signal which carries the image information, by scanning the image recording sheet which has the image information recorded thereon with the stimulating light, by guiding photostimulated luminescence light obtained by the scanning so that the photostimulated luminescence light is incident on the photoconductive layer, and by detecting electric charge generated in the photoconductive layer by the incidence of the photostimulated luminescence light under an electric field applied across the photoconductive layer;
wherein the stimulable phosphor layer of the image recording sheet is stimulated with the stimulating light having a wavelength of 600 nm or greater (preferably, within a red light band of 600 to 800 nm) and emits the photostimulated luminescence light having a wavelength of 500 nm or less (preferably, within a blue light band of 300 to 500 nm);
the photoconductive layer of the solid-state image detector contains amorphous selenium as the main component thereof and also has a smaller area than that of the image recording sheet; and
the stimulating-light scanning means performs the scanning by relatively moving the solid-state image detector on a surface of the image recording sheet.
It is desirable that in the image information reading apparatus of the present invention, the length of at least one side of the solid-state image detector be one fifth or less of the length of one side of the image recording sheet.
It is also desirable that in the image information reading apparatus of the present invention, the thickness of the photoconductive layer of the solid-state image detector be between 0.1 and 100 xcexcm.
In a preferred form of the image information reading apparatus, the solid-state image detector comprises two solid-state image detectors disposed on opposite sides of the image recording sheet, and the image-signal acquisition means detects electric charge generated when the photostimulated fluorescent light emitted from each surface of the image recording sheet is incident on the photoconductive layer of each of the two solid-state image detectors.
In another preferred form of the image information reading apparatus, the solid-state image detector extends lengthwise in a horizontal-scanning direction of the stimulating light, and comprises a plurality of solid-state image detectors disposed in parallel with the horizontal-scanning direction and along a vertical-scanning direction, and the image-signal acquisition means detects electric charge generated when the photostimulated fluorescent light emitted from the image recording sheet is incident on the photoconductive layer of each of the solid-state image detectors.
In the image information reading apparatus of the present invention, it is desirable to dispose an optical filter between the photoconductive layer and the image recording sheet. The optical filter is used for cutting the stimulating light and transmitting the photostimulated luminescence light.
In the image information reading apparatus of the present invention, it is also desirable that the voltage application means applies voltage across the photoconductive layer to generate an electric field under which avalanche amplification is obtained within the photoconductive layer. In this case it is desirable that the thickness of the photoconductive layer in the solid-state image detector is within the range of 10 to 50 xcexcm.
Another preferred form of the image information reading apparatus is further equipped with suppression means for suppressing fluctuations in the image signal caused during acquisition of the image signal, the fluctuations resulting from fluctuations in the electric field being applied across the photoconductive layer.
Furthermore, in accordance with the present invention, there is provided a solid-state image detector comprising a photoconductive layer which exhibits conductibility when irradiated with photostimulated luminescence light emitted from an image recording sheet, wherein the photoconductive layer contains amorphous selenium as its main component and also has a smaller area than that of the image recording sheet. In a preferred form of the solid-state image detector, the length of at least one side of the solid-state image detector is one fifth or less of the length of one side of the image recording sheet.
In another preferred form of the solid-state image detector, the thickness of the photoconductive layer is within the range of 0.1 to 100 xcexcm. In order to effectively obtain avalanche amplification, the thickness may be between 10 and 50 xcexcm.
As described above, the image information reading method and apparatus of the present invention use an image recording sheet having a stimulable phosphor layer that emits photostimulated luminescence light of wavelength 500 nm or less when irradiated with stimulating light of wavelength 600 nm or greater, and also use a solid-state image detector having a photoconductive layer that contains amorphous selenium as its main component. The photoconductive layer of the solid-state image detector has a smaller area than that of the image recording sheet. The sheet is scanned with the stimulating light by relatively moving the solid-state image detector on the sheet surface. Amorphous selenium (xcex1-Se) has a high sensitivity to a blue light band of 500 nm or less, and the quantum efficiency with respect to photostimulated luminescence light in the vicinity of a wavelength of 400 nm may be, for example, as high as 60 to 70% (efficiency at which electric charge is generated is satisfactory). Therefore, combination of amorphous selenium (xcex1-Se) and the stimulable phosphor layer enables efficient reading of the photostimulated luminescence light in a blue light band, emitted from the stimulable phosphor layer. Besides, as the area of the photoconductive layer of the detector is smaller than that of the image recording sheet, there is no generation of excessive dark current and capacitance (output capacitance) can also be made smaller. Thus, a satisfactory S/N ratio can be obtained and a high-quality image can be obtained.
Furthermore, amorphous selenium (xcex1-Se) has hardly any sensitivity to light of wavelength 600 nm or greater and thus transmits the light. Therefore, a ratio of a sensitivity to photostimulated fluorescent light (in the vicinity of a wavelength of 400 nm) to a sensitivity to stimulating light (having wavelengths of 600 to 700 nm) is large. For example, when the film thickness of xcex1-Se is 10 xcexcm with no avalanche amplification, a ratio of a sensitivity to blue light (of wavelength 470 nm) to a sensitivity to red light (of wavelength 680 nm) becomes about 3.5 digits. Note that if the film thickness of xcex1-Se is reduced, the sensitivity to red light will decrease and the blue-to-red sensitivity ratio will further increase. If there is avalanche amplification, the blue-to-red sensitivity ratio will become even greater. Therefore, basically there is hardly any need to use a stimulating light cut filter. If light of wavelength 600 nm or greater which can stimulate a stimulable phosphor layer is used as stimulating light, and the stimulating light is directed onto the stimulable phosphor layer through the xcex1-Se photoconductive layer, the photostimulated luminescence light emitted from the surface of the stimulable phosphor layer can be detected with the photoconductive layer and the picture quality will become satisfactory. In addition, the S/N ratio is satisfactory because xcex1-Se is extremely high in dark resistance, compared with a silicon avalanche photodiode, etc.
If such an electric field with which avalanche amplification operation as charge-doubling operation is obtained is applied across the photoconductive layer, a quantity of electric charge that can be taken out may be exponentially increased. This enhances the S/N ratio of the image signal. Thus, a high-quality image can be obtained.
If fluctuations in the image signal resulting from fluctuations in the electric-field distribution are suppressed, a stable image signal and a higher-quality image can be obtained.
In addition, the solid-state image detector of the present invention is capable of reducing dark current and output capacitance, because it has a photoconductive layer containing amorphous selenium (xcex1-Se) at its main component and makes the area of the photoconductive layer smaller than that of the image recording sheet.
In the case of employing a silicon avalanche photodiode, it is difficult to make a zero-dimensional detector or one-dimensional detector (line sensor) long, since silicon is a crystal and it is difficult to make it long. However, amorphous selenium (xcex1-Se) does not get shocked so easily as a sensor employing glass and is suitable for solidification and can be thinned, because a low-temperature deposition process can be performed on xcex1-Se. For example, it is easy to manufacture a solid-state image detector as short as 17 inches (about 25. 4 mm) in length.